Energy-dense triglycerides from oil seeds or microalgae have the potential to at least partially displace petroleum-derived fuels. However, triglycerides cannot be used directly in current combustion engines due to their low volatility and high viscosity. Vegetable oils, the primary source of triglycerides that are currently used for biofuel production, are typically converted to usable fuels through transesterification. In this process, triglycerides react with methanol, usually in the presence of a homogenous catalyst (e.g. sodium hydroxide or methoxide), to produce a mixture of fatty acid methyl esters (FAMEs; commonly known as biodiesel). Since the transesterification reaction is reversible, excess alcohol is required to shift the reaction towards ester production; thus, unreacted alcohol along with catalyst and glycerol need to be separated and recovered at the end of the reaction. Also, the presence of free fatty acids in the feedstock leads to saponification, which results in the loss of catalyst and a decrease in the yield of esters. Finally, while biodiesel is compatible with petro diesel, it exhibits a relatively high melting point (which limits its usage in cold climate regions) and lower oxidative stability compared to diesel.
As an alternative, pyrolysis (thermal cracking in the absence of O2) of vegetable oil can produce hydrocarbons that are compatible with a variety of petro-fuels such as gasoline, jet, and diesel. Studies on pyrolysis of seed oils, such as canola, palm, and soybean oil, reveal that the primary compounds in the product are paraffins, olefins, carboxylic acids, and small amounts of aromatics. Although a mixture is produced, the pyrolysis products can be separated (e.g., using distillation) and used either directly or processed by common refinery methods such as hydrogenation, hydrotreatment, or alkylation to obtain gasoline- and/or diesel-like fuels. Further, unlike transesterification, conversion of triglycerides by pyrolysis avoids the use of methanol and unrecoverable homogenous catalysts. In addition, the un-degraded long chain fatty acids from pyrolysis (e.g. oleic acid) can be separated from hydrocarbons to serve as feedstock for oleochemical production.
Pyrolysis of vegetable oils has typically been carried out in batch reactors at a temperature range of 300-500° C. and atmospheric pressure. However, batch processes are not appropriate for commodity scale industrial operations due to low throughput. In addition, most batch studies report low yield of liquid products, likely due to the high residence time in these closed systems which allows secondary reactions of primary products to low molecular weight (C1-C4) noncondensable gases. For example, batch pyrolysis of tung oil has been conducted at 450° C. and a residence time of 100 min, and the result was only 55% yield of liquid products. Batch pyrolysis of canola and soybean oil at 440° C. has also been conducted, but at a lower residence time of 10 min and also with high hydrogen pressure (2200 kPa). This approach resulted in higher organic liquid product (OLP) yields (67% OLP for soybean oil and 61-69% for canola oil), likely due to the shorter reaction time and in situ hydrogenation of unsaturated primary products that lowered the production of secondary noncondensable gases.
Others have attempted pyrolyzing vegetable oil in continuous flow reactors. However, due to high reaction residence time, low yield of liquid products has been reported from these attempts. In some cases, carrier gas was used to decrease the residence time. However, carrier gas usage increases operating (i.e., cost associated with supplying and heating the carrier gas) and capital (i.e., larger reactor and condenser) costs.
For example, thermal cracking of canola oil has been performed in an effort to develop a continuous process for vegetable oil pyrolysis. The reactor consisted of a fixed-bed of inert materials (ceramic and quartz glass chips) that was kept at 500° C. Due to the high reactor temperature and surface area created by inert particles, feed likely vaporized and cracked simultaneously. However, the residence time was still high (18 min) and only 15% of feed was recovered as OLP. Nearly 75% of feed was lost as uncondensed gases such as small chain hydrocarbons (C1-C4) and H2.
Without wishing to be bound by theory, OLP yields from vegetable oils are understood to be better in continuous reactors with low residence times, since secondary reactions are generally minimized due to continuous product removal and condensation, similar to the fast pyrolysis of solid substrates (e.g., biomass and coal), which are carried out at high temperature and short residence time to maximize liquid products. Theoretical reaction mechanisms also indicate that liquid formation would be more favorable at low residence time. Some have explored this approach and have reported improved liquid product yields. For instance, in previously reported continuous pyrolysis of waste fish oil a relatively high yield of 72% liquid products was observed at reaction temperature of 525° C. and low vapor residence time of 24 s. In addition to short residence time, a high free fatty acid content in the feed also possibly contributed to the high yields of OLP since fatty acids are more amenable to thermal cracking than triglycerides. However, while yields were improved, nearly 30% of feed material was still lost to uncondensed gases. An additional drawback of the reaction system was the requirement of preheating the feed. Since the objective was to quickly vaporize the feed in the reactor (for short residence time), the feed was preheated to 475° C. Thus, while the pyrolysis residence time was short, the overall time period for which feed was exposed to high temperature was still large and could have possibly resulted in some oil degradation during preheating. Furthermore, including a preheater also increases the capital cost of the reaction system.
Liquid product yields from known methods are typically in the range of 20-70%, depending on various factors such as reaction conditions and feedstock. Liquid products, rather than gas, are more desirable, since they have higher heating values on a volumetric basis and are easier to store and transport. Although pyrolysis of vegetable oil allows the direct production of hydrocarbons fuels, this technology has not yet achieved the high liquid yields necessary for commercial success. It would therefore be desirable to discover a process or system for achieving such yields.